Metal oxides have long generated technological and industrial interest because of their very diverse properties (optical, electrical, magnetic, etc.), combined with their overall characteristics of hardness, thermal stability and chemical resistance. Materials with high ion mobility have a promising future as solid electrolytes and cathodes for batteries. In addition, metal oxides, most specifically transition metal oxides, belong to a class of widely used catalysts. They exhibit acidic or basic properties, which make them appropriate as catalysts or catalyst supports.
Various techniques are used in the production of oxide materials. Precipitation and co-precipitation of ions from solutions have been used for many years in most industrial production of fine powders for ceramics or catalyst supports. Precipitation is the result of a process of inorganic polycondensation involving the hydrolysis of metal ions in solution and the condensation of hydroxylated complexes. Hydrolysis, condensation and complexation reactions of cations in aqueous solution are the phenomena involved in the formation of the solid by precipitation.
Sol-gel processes are a precipitation-based synthetic method of metal oxides in which a network is formed throughout the body of a solution of a metal (e.g., in the form of or derived from a salt or complex) by a progressive change of the solution into a gel (colloidal solution of hydrous metal oxide nanoparticles), then in most cases to a dry network. In comparison to other precipitation-based synthetic methods in which solid particles are precipitated and separated from the solution, sol-gel processes can be advantageous because the chemical composition of the products can be controlled more conveniently due to the nominal composition relatively homogeneous throughout the gel structure.
The acidity of the precursor solution influences gel formation, as it determines the nature of the predominant species of the metal complexes and the mechanism of hydrolysis and polycondensation. Conventionally, sol-gel processes are initiated/controlled by purposely changing pH of the precursor solution.
Inorganic salts and complexes of transition metals, especially the early transition metals (e.g., the metals of Groups 3 to 6) and of Group 13 metals (e.g., B and Al) are distinguished from others by greater chemical reactivity resulting from the lower electronegativity of the metals and their ability to exhibit several coordination states, so that coordination expansion occurs spontaneously upon reaction with water or other nucleophilic reagents. These characteristics lead to rapid or uncontrollable hydrolysis and/or polycondensation of the metal ions in an aqueous solution, hindering formation of a gel. In order to inhibit the premature uncontrolled precipitation, organic chelators such as citrates, acetonates or oxalates have been used to form polymeric gel precursors. Inorganic competing ions such as oxyanions or metal ions are known to slow the polycondensation rate to achieve the uniformity of gel formation throughout the solution.